A series of copper zinc oxide catalysts have been prepared using a co-precipitation procedure by varying the ageing time in the precipitation liquor. The catalysts were studied for the oxidation of CO at ambient temperature. All the catalysts exhibited ambient temperature CO oxidation activity and the ageing time is important in controlling the catalyst activity. The most active catalysts were aged for 165 min. Characterisation by powder X-ray diffraction showed that the catalyst precursors were sensitive to ageing time. It was also evident that ageing time influenced the precursor morphology. The calcined catalysts were all comprised of CuO and ZnO. The origin of the high activity for the CuO/ZnO catalysts prepared with short ageing times is not fully understood, but it appears that the highly dispersed CuO and ZnO, and the presence of solid solution phases formed during the controlled precipitation and ageing process are important. (C) 2003 Elsevier B.V All rights reserved.